Bump version and update changelog.

Version 0.7.0

* Add serde support (and crate feature)
* Implement fmt::Display instead of ToString

Version 0.6.0

* [SECURITY] Replace the `chrono` crate with `time` (#1)

Version 0.5.3

* Add homepage to cargo metadata

Version 0.5.2

* Improve docs and readme

Version 0.5.1

* Initial release